Previous devices and methods of positioning elements such as valves, utilize a DC motor controller and a DC motor to develop the required torque necessary to position the mechanical load. Time proportional control of an induction motor using magnetic contactors is another known method.
The inventive device and method uses a three phase variable speed constant torque induction motor controller and a three phase induction motor. The driving element of the servo positioner is an electric motor. Previous technology has dictated that the control of a permanent magnet DC motor is more economical than a three phase induction motor for horsepower ratings below 10. Three disadvantages associated with DC motors are large size for HP rating, necessary routine maintenance, and high cost. An AC three phase induction motor reduces all these disadvantages which are associated with the DC machine. An AC induction motor is approximately 1/3 the size of an equivalent DC motor, 20% of the cost, and requires little or no maintenance due to its having only one moving part.
The disadvantage of an AC induction motor is the increased complexity of the control electronics. Until recently, the control of less than 10 HP induction motors has been uneconomical due to the large number and cost of the associated electronic components. The past few years have shown a significant reduction in cost of power electronics and complex LS1 circuits. The current trend of price reduction is expected to continue as the semiconductor industry improves its processes. Consequently, AC induction control has become more desirable than DC motor control in the 1 to 10 HP range. The proposed application requires a three phase induction motor rated at 3 horsepower or less.
The use of a microprocessor based controller has, according to the invention, in addition, improved the performance and flexibility of the control electronics.
Previous methods of controlling an induction motor utilize analog circuitry consisting of sinusoidal and triangular wave forms which are generated to produce the pulse width modulated wave forms necessary to control the motor for constant torque variable speed control. The analog technique is usually complex, requiring numerous factory and field adjustments.
Digital techniques including microprocessors have been attempted for open loop induction motor speed control.
U.S. Pat. No. 4,099,109 to Abbondanti discloses a digital apparatus for synthesizing pulse width modulated wave forms. According to that reference, however, the wave forms are selected by hardware. The versatility of Abbondanti for adapting the induction motor control to various applications is thus limited. In addition the provisions of the required logic for the induction motor control using hardware limits the flexibility of such control, and the adaptability thereof to various different conditions and requirements.